Battery-powered host devices (hereinafter “host devices”) such as laptop personal computers (PCs) and personal digital assistants (PDAs) have become increasingly popular as their size and weight have decreased, while great strides have been made in their functionality and battery life. New sales of host devices are driven in part by portability, which depends substantially on the availability of a long battery life. Accordingly, there is significant industry pressure to increase battery life in these host devices.
Presently, host devices are often equipped with electrical devices such as telephone modems. Typically, the electrical devices are powered by the host device to which they are attached, e.g., a laptop PC. When an electrical device is in use, it presents a significant power drain on the battery of the host device, thereby decreasing the battery life of the battery of the host device considerably.
One method for increasing battery life in the host device is to use power from a telephone line to provide a portion of the power required by an electrical device such as a modem. This is possible because telephone lines in the United States and elsewhere can have voltages of over 100V. An example of a telephone line powered electrical device designed to operate off of power derived from the telephone line is a line powered modulator circuit disclosed in commonly assigned U.S. patent application Ser. No. 09/280,473, filed on Mar. 30, 1999, entitled Method and Apparatus for Decreasing Distortion in a Line powered Modulator Circuit, incorporated fully herein by reference.
Existing methods and devices that use power from the telephone lines, however, have many limitations including the following: (1) they power only a portion of the electrical device, such as a modulation circuit within a modem, thereby failing to realize the potential increase in battery life associated with powering the entire electrical device from the telephone lines; (2) they exclusively use power from the telephone lines, thereby becoming vulnerable to telephone line power differences; and/or (3) they are designed to operate at the voltage levels of the telephone line, thereby requiring special circuitry to operate at the relatively high telephone line voltage level of 100V rather than at the typical voltage level of circuitry within the host device, e.g., 5V or less.
Accordingly, there is a need for methods and devices for drawing power from a telephone line to supply all of the power required by an electrical device such as a telephone modem, that can withstand telephone line power fluctuations, and that is suitable for use with existing circuitry. The present invention can fulfill this need among others.